The disclosure relates in general to Magnetoresistive Random Access Memory (MRAMs), and more particularly to methods and devices to determinate writing current for MRAM cells.
Magnetic random access memory (MRAM) cells are often based on a magnetic tunnel junction (MTJ) cell. Basically, an MTJ configuration can be made up of three basic layers, a “free” ferromagnetic layer (free layer), an insulating tunneling barrier, and a “pinned” ferromagnetic layer (pinned layer). In the free layer, the magnetization moments are free to rotate under an external magnetic field, but the magnetic moments in the “pinned” layer cannot. The pinned layer can be composed of a ferromagnetic layer and/or an anti-ferromagnetic layer which “pins” the magnetic moments in the ferromagnetic layer. A very thin insulation layer forms the tunneling barrier between the pinned and free magnetic layers. In order to detect states in the MTJ configuration, a constant current can be applied through the cell. As the magneto-resistance varies according to the state stored in the cell, the voltage can be detected over the memory cell. To write or change the state of the memory cell, an external magnetic field can be applied, sufficient to completely switch the direction of the magnetic moments of the free magnetic layers.
MTJ configurations often employ the Tunneling Magneto-Resistance (TMR) effect, which allows magnetic moments to quickly switch the directions in the magnetic layer by an application of an external magnetic field. Magneto-resistance (MR) is a measure of the ease with which electrons flow through the free layer, tunneling barrier, and the pinned layer. A minimum MR occurs in an MTJ configuration when the magnetic moments in both magnetic layers have the same direction or are “parallel”. A maximum MR occurs when the magnetic moments of both magnetic layers are in opposite directions or are “anti-parallel.”
FIG. 1A is a schematic perspective view illustrating a conventional MTJ memory cell of an MRAM device. FIG. 1B is a schematic perspective view illustrative of read out operation of the MTJ memory cell of FIG. 1A. FIG. 1C is a plane view illustrative of respective magnetization states depending on stored data of the MTJ memory cell of FIG. 1A.
A single memory cell comprises first operative layer 11, pinned layer 12, tunnel barrier layer 13, free layer 14, and second operative layer 15. The MTJ memory cell comprises pinned layer 12, dielectric layer 13, and free layer 14. The tunnel barrier layer 13 is sandwiched between pinned layer 12 and free layer 14. The pinned layer 12 is in contact with the first operative layer 11. The free layer 14 is in contact with the second operative layer 15. The pinned layer 12 and the free layer 14 are ferromagnetic materials. The dielectric layer 13 is an insulating material. The pinned layer 12 has a fixed magnetization direction. The dielectric layer 13 has a thickness of about 1.5 nanometers. The free layer 14 has a thickness of about 20 nanometers. The free layer 14 has a freely changeable magnetization direction.
The magnetization direction of the free layer 14 indicates stored data. The free layer 14 serves as a data storage layer. The first operative layer 11 and the second operative layer 15 extend in directions perpendicular to each other. The MTJ memory cell is positioned at a crossing point between the first operative layer 11 and the second operative layer 15. In FIG. 1B, a current 16 flows from the first operative layer 11 to the second operative layer 15 through the pinned layer 12, the dielectric layer 13, and the free layer 14. The MTJ memory cell is capable of storing binary digit data “0” and “1”. If the magnetization directions of the pinned layer 12 and the free layer 14 are parallel to each other, the MTJ memory cell stores a first binary digit, for example, “0”. If the magnetization directions of the pinned layer 12 and the free layer 14 are not parallel, the MTJ memory cell stores a second binary digit, for example, “1”. The magnetization direction of the free layer 14 changes depending on an externally applied magnetic field.
Electrical resistance of the dielectric layer 13 varies by about 10–60% due to tunneling magnetoresistance effect between a first state where the magnetization directions of the pinned layer 12 and the free layer 14 are parallel and a second state where the magnetization directions of the pinned layer 12 and the free layer 14 are not parallel. A predetermined potential difference or a predetermined voltage is applied to the first and second operative layers 11 and 15 to apply a tunneling current from the pinned layer 12 to the free layer 14 through the dielectric layer 13. This tunneling current varies depending on the variable electrical resistance of the dielectric layer 13 due to tunneling magnetoresistance. Data can be retrieved from the MTJ memory cell by detecting the variation in the tunneling current.
FIG. 2A is a fragmentary schematic perspective view illustrative of an array of MTJ memory cells. FIG. 2B is a fragmentary schematic perspective view illustrative of the array of the MTJ memory cells during the operation shown in FIG. 2A.
The first operative layers 11 extend parallel to each other in a first direction. The second operative layers 15 extend parallel to each other in a second direction, perpendicular to the first direction. The single first operative layer 11 and the single second operative layer 15 have a single crossing point, where a single MTJ memory cell “C” is provided. The first operative layers 11 and the second operative layers 15 have an array of crossing points where plural MTJ memory cells “C” are provided. The first operative layers 11 serve as word lines. The second operative layers 15 serve as bit lines. One of the plural MTJ memory cells “C” is selected by selecting one of the word lines and one of the bit lines, for read or write operations to the selected MTJ memory cell “C”.
The MRAM has the array of the MTJ memory cells, each of which comprises the tunneling magnetoresistance element utilizing the tunneling magnetoresistance effect, wherein the tunneling magnetoresistance element includes an insulating thin film sandwiched between the two or more ferromagnetic thin films. The tunneling magnetoresistance element is switched between a first state, in which the magnetization directions of the two ferromagnetic thin films are parallel to each other, and a second state, in which the magnetization directions of the two ferromagnetic thin films are anti-parallel. The resistance of the insulating film, detected the tunneling current, is different for the first and second states. These two states correspond to binary digits, for example, the first state can correspond to data “0”, and the second state to data “1”.
The write operation is accomplished as follows. One of the word lines 11 and one of the bit lines 15 are selected. A first write current Isw is applied to the selected word line 11s. A first magnetic field Msw is generated around the selected word line 11s. The first write current Isw has a predetermined current value and a predetermined direction. A second write current Isb is applied to the selected bit line 15s. The second write current Isb has a predetermined current value and a predetermined direction. A second magnetic field Msb is generated around the selected bit line 15s. As a result, a superimposed magnetic field of both the first and second magnetic field Msw and Msb is applied to the crossing point of the selected word line 11s and the selected bit line 15s. The selected MTJ memory cell “Cs” is positioned at the crossing point of the selected word line 11s and the selected bit line 15s, for which reason the selected MTJ memory cell “Cs” is applied with the superimposed magnetic field. The free layer of the selected MTJ memory cell “Cs” is also applied with the superimposed magnetic field, whereby magnetic domains of the free layer become ordered in a first direction, for example, parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ memory cell “Cs” stores a binary digit data “0”.
Any first write current Isw or second write current Isb changes its current direction to an opposite direction, whereby the direction of the magnetic field is inverted, and the direction of the superimposed magnetic field is changed by approximately 90 degrees. As a result, the magnetic domains of the free layer become ordered in a second direction, for example, in a direction anti-parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ memory cell “Cs” stores another binary digit “1”.
The read operation is accomplished as follows. One of the word lines 11 and one of the bit lines 15 are selected. A potential difference is applied between the selected word line 11s and the selected bit line 15s for measuring a current value to detect a resistance value of the selected memory cell “Cs” to the tunneling current. Namely, a predetermined potential difference or a predetermined voltage is applied to the selected word line 11s and the selected bit line 15s to provide a tunneling current from the pinned layer through the insulating layer to the free layer of the selected memory cell “Cs”. This tunneling current varies depending on the variable electrical resistance of the insulating layer due to the tunneling magnetoresistance effect. The binary digit data can be detected from the selected memory cell “Cs” by detecting the variation in the tunneling current.
As described, the ability of this type of cell to store electrically accessible data hinges on electron tunneling between the free layer and the pinned layer, which in turn is dependent on the relative directions of magnetization of these two regions. Rotating the magnetization in the free layer into one of at least two selectable directions results in binary state stored in the cell. If the cell is oriented with its magnetic easy-axis (“EA”) horizontal, an electrical writing current through a vertical line will apply an EA magnetic field, and a current through a horizontal line will apply a hard-axis (“HA”) magnetic field, to the cell.
In one implementation of MRAM cells, the writing of individual cells adheres to a concept referred to as the “asteroid” for switching. The switching threshold of a single free region depends on the combination of EA and HA magnetic fields applied thereto. This “Stoner-Wohlfarth” asteroid model, shown in FIG. 3A, illustrates these threshold values in the plane of applied EA and HA fields. Switching occurs when a combination of EA and HA fields at the cell results in a vector outside of the asteroid curve. Vectors inside the asteroid curve will not switch the cell from one of its current bi-stable states. This asteroid model also illustrates how the EA field needed to switch a device is reduced in the presence of an HA bias field. Selectively switching a single cell within the array is achieved by applying electrical currents through a selected pair of horizontal and vertical lines. These currents generate a combination of EA and HA fields only at the cell located at the intersection of these lines, theoretically switching the selected cell, but not the neighboring cells.
All the cells along the horizontal line experience the same applied HA field. Similarly all the cells along the vertical line experience the same applied EA field. However, only the cell at the intersection of these lines experiences the combination of both fields necessary for switching.
Problems arise when the thresholds of the asteroid curve vary from cell to cell, and from hysteresis loop to hysteresis loop in the same cell. This leads to a broadening of the asteroid into a band of threshold values as shown in FIG. 3B. Since the ability to selectively switch cells hinges on all cells except one along a line not being switched under a single applied HA or EA field, if this band of the asteroid curve broadens excessively, it is no longer possible to selectively write individual cells, with equivalent writing stimuli, since other non-selected cells along the lines will also switch. In other words, if the magnetic field generated by writing lines is too weak or the switching field for the magnetoresistive cell is too large, writing operation cannot be successful. In addition, if the magnetic field generated by writing lines is too strong or the switching field for the magnetoresistive cell is too small, writing disturb occurs on the on-wanted cells.
Whether using the above-discussed asteroid selection model, or any other selection model, a major challenge in the successful implementation of an MRAM array with effective cell selectivity is the fabrication of many memory cells with nearly identical electrical and magnetic properties. This is particularly difficult for magnetic devices since their response is sensitive not only to local defects but also to edge or surface roughness.